dampening computer/hd noise

my computer sits on an upturned yellow plastic milk carton beside my chair and desk
and the Barracuda hard drive - i believe - was making a thrummmmarrrr-thrummmmarrrr
oscillating noise, felt as much as heard

i seem to have reduced a lot but not all of the thrumming by placing a piece of 3/8th"
dense foam between the computer and carton - foam is the stuff bought at a camping
supply store to use as a mattress when back-packing/camping

To follow Scat; you need to try to create a silent system from scratch.

Use a SSD, and make sure that the power supply is either fanless or has a fan with low noise. Always swap out the PSUs you get with a case with a choice one to avoid noise. You should also get a CPU fan that's very quiet. Last, make sure not to overclock your CPU - sacrifice power/speed and receive silence in return. Silentpcreview.com is one site to look for tests and parts.